Hi
I am designing a boost converter for my college final year project .The boost converter would take input of 12 volts and give variable outputs from 20 volts to 60 volts.The design is a bit ready the only problem i am facing is regarding the pwm controller. I tried to design the pwm using the 555 ic but the problem is that when i am trying to change the duty cycle the frequency is also getting changed. MAX038 and ICL8038 was the other two alternatives available but both the are now not in production can you please tell me some other way to design the pwm controller that would have independent frequency and duty cycle control in it or only a fixed frequency but variable duty cycle control in it.
My maximum frequency requirement will be near about 30 khz and an adjustable duty cycle from 2% to 98%.

Check out the 555 circuits available on this forum. You're looking for the 555 astable circuit that uses 2 diodes to separate the charge and discharge paths of the timing capacitor. This allows independent control of frequency and duty cycle.

If you need help finding it here, come back and someone will know where it is.

Is there a reason you are not using a boost converter IC, or at least a PWM IC? Are these parts forbidden by the project?

You can get independent frequency and duty cycle by using two 555s. One is an astable running at a constant 30 KHz. It triggers the other 555 that is a variable monostable that sets the pulse width.

Many power supply controller ICs use a different method. Start with a 30 KHz triangle-wave oscillator. That goes into a comparator with a variable trip level. As the trip point varies up and down, the output pulse width varies. In terms of overall circuit complexity, this is much easier than varying a 555 pulse width based on analog feedback voltage from the power supply output.

Maybe you could add a bit to your spec.
What is the current requirement? At 30Khz the inductor could get quite large.
The duty cycle should be from about 50% to 80%. Why such a broad range?
Is the current constant or will it vary as well as the voltage?

Hi AnalogKid
Using of boost converter ics are strictly prohibited in our project.
Can you please help me out with any kind of pwm ics with duty cycle variation at constant frequency.
Can you help me with the circuitry that involves two 555 ics or circuitry of any power supply controller ICs with the above mentioned features.

Hi wayneh
I have gone through that circuit earlier but again the main problem with that circuit is also that the frequency is not completely stable I need a circuitry that will have completely independent frequency and duty cycle variations.

Astable multivibrator running at 30kHz ,output is differentiated and passed to trigger of another 555 configured in Monostable operation and the Voltage at the Control Voltage pin of the 555 could be varied to change the pulse width by means of a pot or DAC hooked up to a micro. The PWM output is connected to the base of a transistor. I tried to make a similar one just now.

I heard a humming sound from the inductor, maybe because it was switching in the audible freq range? Also I burnt my 2n2222 by connecting a 12v 2A battery as power supply. What if I connect a resistor between emitter and ground and take voltage across it?

Is there a reason you are not using a boost converter IC, or at least a PWM IC? Are these parts forbidden by the project?

You can get independent frequency and duty cycle by using two 555s. One is an astable running at a constant 30 KHz. It triggers the other 555 that is a variable monostable that sets the pulse width.

Many power supply controller ICs use a different method. Start with a 30 KHz triangle-wave oscillator. That goes into a comparator with a variable trip level. As the trip point varies up and down, the output pulse width varies. In terms of overall circuit complexity, this is much easier than varying a 555 pulse width based on analog feedback voltage from the power supply output.

Hi wayneh
I have gone through that circuit earlier but again the main problem with that circuit is also that the frequency is not completely stable I need a circuitry that will have completely independent frequency and duty cycle variations.

Click to expand...

Then take a look at Figs 5.3 and 10.4 here. They give a simple way to get separate control of frequency and duty cycle using very common components.